This question has been done to death, but basically internship is normally done in your penultimate year and during the summer for a couple of months. On the other had a cadetship is first/second year (obtained in year 12 or first year) and involves working full-time whilst studying part time for a portion of your degree. It also lasts for 2 years so you'll gain much more experience. On top of this you may receive an allowance for books and other necessary study materials which isn't given for an internship. The cadetship basically means you'll have 2 years of relevant full time work experience contributing to your CA requirement when you graduate from your 3 year degree (which would have taken 4 due to part time study).